1. Field of the Invention
The present invention relates to an automatic transmission primarily intended for motor vehicle use, and more particularly to an "adaptive" transmission that is controlled electronically and hydraulically.
2. Description of Related Art
In general, the major components featured in an automatic transmission are: a torque convertor, fluid pressure-operated multi-plate drive or brake clutches and/or brake bands which are connected to individual elements of planetary gear sets in order to perform gear shifts without interrupting tractive power. The control system of the automatic transmission is typically hydraulically operated through the use of several valves to direct and regulate the supply of pressure. This hydraulic pressure control will cause either the actuation or deactuation of the respective clutches and/or bands for effecting gear changes in the transmission.
However, in recent years, a more advanced form of transmission control system has been developed, which enables the transmission to adapt itself to changing conditions. In this regard, U.S. Pat. No. 4,875,391, issued on Oct. 24, 1989 to Leising, et al., sets forth a fundamental development in this field. Specifically, this patent discloses an automatic transmission which features an "adaptive" control system that includes an electronic controller and electrically operated solenoid-actuated valves for controlling certain fluid pressures for effecting gear changes in the transmission.
One characteristic of four-speed transmissions is that the transmission may hunt between third and fourth gear while the vehicle ascends a moderately steep grade. When the vehicle starts up the grade and begins to lose speed in fourth gear, the driver, in an attempt to maintain speed, depresses the accelerator pedal enough to make a fourth to third gear kickdown shift. In third gear, the vehicle accelerates. The driver would then lift his foot, and a third to fourth gear upshift would be triggered. As a result, the transmission constantly or repeatedly hunts between third and fourth gear which is undesired.